September is a busy month for Earth observation satellites, and so here is a round-up of the month.

Launches Gone
The Indian Space Research Agency (ISRA) launched the INSAT-3DR weather satellite on September 8th into a geostationary orbit. It carries a multi-spectral imager capable of collecting data in six wavebands: visible, shortwave and midwave infrared, water vapour and two thermal bands. Taking an image every 26 minutes it will be used to monitor cloud patterns and storm systems, collecting data about outgoing longwave radiation, precipitation estimates, Sea Surface Temperature (SST), snow cover and wind speeds.

The second major launch took place on September 15th, from Europeâ€™s Space Centre in French Guiana, when five new Earth observation satellites were put into orbit.

Four of these satellites, SkySats 4, 5, 6 & 7, were launched for the commercial company Terra Bella â€“ which is owned by Google. Itâ€™s reported that they have informally named these satellites after the Star Wars characters: R2D2, Luke, C3PO and Leia! These small satellites provide 90 cm resolution for panchromatic images and 2 m for visible and near infrared wavebands. They also offer video acquired at 30 frames per second with a resolution of 1.1 m.

In addition, this launch brought a new country into the Earth Observation satellite owning family, as Peru launched PeruSAT-1 which will be operated by their military authorities. This satellite is in a 695 km sun-synchronous low Earth orbit and will provide imagery in the visible light wavebands with a 70 cm resolution. The data is expected to help study forest health, monitor illegal logging and gold mining, and provide support with natural disasters. However, the details of who can access to the data, the cost and how to access it are still to be made public.

Launches to ComeLast week we said DigitalGlobeâ€™s WorldView-4 satellite was due to launch on the Friday. The problem of having a blog go live before an event means you can be wrong, and on this occasion we were! Fridayâ€™s launch was postponed for two days due to a leak during the propellant loading. Unfortunately, a wildfire then broke out near the Vandenburg Air Force base, and the launch had to be postponed a second time. It is hoped it will go ahead before the end of the month.

Following on from INSAT-3DR, ISRA is due to launch another four satellites in the last week of September including:

Indiaâ€™s ScatSat, a replacement for the Oceansat-2. Carrying OSCAT (OceanSat-2 Scanning Scatterometer) it will offer data related to weather forecasting, sea surface winds, cyclone prediction and tracking satellite. The data collected will be used by organisations globally including NASA, NOAA and EUMETSAT.

A second Earth observation satellite on the launch is Algeriaâ€™s first CubeSat – AlSat Nano. It was designed and built at the Surry Space Centre by Algerian Graduate students, as part of joint programme between the UK Space Agency and the Algerian Space Agency. It will carry a camera, magnetometer and will be testing an innovative solar cell which is one tenth of a millimetre thick.

Launches Being Planned
The next country to join the Earth Observation community could well be North Korea. It was reported this week that they had carried out a successful ground test of a new rocket engine which would give them the capacity to launch various satellites, including Earth Observation ones.

Pictures showing Sentinel-1Aâ€™s solar array before and after the impact of a millimetre-size particle on the second panel. The damaged area has a diameter of about 40 cm. Data courtesy of ESA>

Space is unpredictable. Things donâ€™t always go as planned. Over the last few weeks some of the difficulties of working in space have been highlighted.

Gaofen 10
The start of September did not go well for the satellite industry with two failed launches. Firstly, the Chinese Gaofen 10 Earth observation satellite launched on the 31st August onboard the Long March 4C rocket did not appear to have achieved its orbit. The lack of certainty about this is because no official announcement has been made by Chinese authorities, despite pictures of debris appearing on social media the following day. Gaofen-10 was believed to be carrying a multi-polarized C-band SAR instrument and was intended to be part of the China High-Resolution Earth Observation System (CHEOS), joining the existing seven orbiting Gaofen satellites to provide real-time global Earth observations.

SpaceX
The explosion of the SpaceX Falcon rocket on the Cape Canaveral Launchpad received significantly more mainstream media attention than Gaofen 10. This was partly due to the fact it was a SpaceX rocket, and partly because the satellite it carried was going to be used by Facebook. When you have two of the USâ€™s most well-known technology gurus involved, it was bound to grab the headlines.

No-one was hurt, but the satellite was destroyed by the explosion that occurred whilst the rocket was being loaded with fuel; investigations continue into the cause of this. It was an Israeli communication satellite called Amos 6, whose main purpose was the delivery of television channels. However, Facebook also had an agreement to use the satellite to provide internet connectivity to sub-Saharan Africa.

Sentinel-1A Struck in Space
ESA recently confirmed that the Copernicus Sentinel-1A satellite was hit by a millimetre-size particle on one of its solar wings on the 23rd August. The impact caused slight changes to the orientation and orbit of the satellite, although it hasnâ€™t impacted performance.

Engineers were able to activate the onboard cameras, which provided a clear picture of the impact site on the solar panel, which can be seen in image at the top of the blog. The damaged area is approximately 40 centimetres wide, which is consistent with the impact of a fragment of less than 5 millimetres. This damage has reduced the power generated by the solar wing, although the loss will not impact performance as current power generation remains higher than what the satellite requires for routine operations.

Itâ€™s not clear whether Sentinel-1A was stuck by space debris or a micrometeoroid. Given the amount of space debris up there significantly larger than 5 millimetres, the potential damage that could be done to satellites is massive!

Back in STEREO
On a more positive note, last month NASA re-established contact with a satellite after a gap of almost two years. In 2006 NASA launched a pair of twin Solar TErrestrial RElations Observatory (STEREO) satellites to provide data about the sunâ€™s solar flares and coronal mass ejections. Contact was lost with STEREO-B (so called because it was orbiting behind STEREO-A; the A signified it was ahead!) on the 1st October 2014 during a routine test. Since that time NASA has been working to re-establish contact with STEREO-B, and amazingly did so on the 21st August 2016!

Having made contact the team are assessing the satellite, and its components, with the hope of bringing it back to working order in the near future.

Close-up of the Philae lander, imaged by Rosettaâ€™s OSIRIS narrow-angle camera on 2 September 2016 from a distance of 2.7 km. The image scale is about 5 cm/pixel. Philaeâ€™s 1 m-wide body and two of its three legs can be seen extended from the body. Image courtesy of ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/ INTA/UPM/DASP/IDA.

Philae Located!
A second discovery after lost contact is ESAâ€™s Philae Lander! This was the robot that made a historic landing on Comet 67P/Churyumovâ€“Gerasimenko in November 2014, as part of the Rosetta mission. Unfortunately, Philae bounced away from the intended landing site and after a short period of operation, communications were lost. There was brief resurrection in July 2015, before silence returned.

Amazingly, last week the resting site of Philae was finally located with Rosettaâ€™s high resolution camera. It is stuck in a dark crack on the comet surface, explaining why its solar powered batteries were unable to be recharged.

Philae will be joined later this month by the Rosetta probe itself, as it will be crash landed onto the comet. Cameras and chemical sensors will be operating throughout the descent which is planned to take place on the 30th September bringing to end this historic comet chasing mission.

Onward Despite Difficulties
DigitalGlobeâ€™s WorldView 4 satellite is due to be launched on Friday, 16th September aboard an Atlas V rocket from Vandenberg Air Force Base. Like WorldView 3 this satellite should provide imagery with a spatial resolution of 31 cm in panchromatic mode and 1.24 m in multispectral mode.

This shows that despite all of the ups and downs of the last few weeks, the satellite industry keeps moving forward!

Spatial resolution is a key characteristic in remote sensing, where itâ€™s often used to refer to the size of pixels within an acquired image. However this is a simplification as the detector in the satellite doesnâ€™t see the square suggested by a pixel, but rather it sees an ellipse due to the angle through which the detector receives the signal – known as the instantaneous field of view. The ellipses are turned into square pixels by data processing in creating the image.

The area of the port of Rotterdam shown using a Landsat image (background) at 30m resolution and MERIS full resolution image (inset image) at 300m resolution; data courtesy of the USGS and ESA. Example used within Hydrographic Academy eLearning material.

Therefore, for example, when viewing an image with 1km resolution not only will you not be able to see anything that is smaller than 1km in size, but objects needs to be significantly larger than 1km for any detail to be discernable. Whilst this might be fine if you looking at changes in temperature across the Atlantic Ocean, it wonâ€™t be much use if you are interested in suspended sediment blooms at the mouth of a small river.

Any image with a spatial resolution of between 50m and 1km, is described as having low spatial resolution. For example, MODIS operates low spatial resolutions ranging from 250m to 1000m as the primary focus is global mapping rather than capturing detailed imagery for local regions.

If you want to look for smaller objects, youâ€™ll need use images with medium spatial resolutions of between 4m to 50m. There is quite a lot of freely available imagery within this range. For example, NASAâ€™s Landsat 8 operates at 15, 30m and 100m resolution and ESAâ€™s Sentinel-1A operates at the three resolutions of 5m, 20m and 100m. If you want go even finer, you will require high spatial resolution images that go down to resolutions of between 4m and 1m, or very high spatial resolution images which cover the 0.5m â€“ 1m range. Commercial organisations tend to operate satellites with these higher levels of resolution, and they charge for making the images available. Itâ€™s likely that military satellites offer imagery down to 0.15m, but there are regulations in place to prevent the sale of extremely high resolution imagery as itâ€™s considered to be a potential danger to security.

Spatial resolution was in the headlines last week with launch of the DigitalGlobeâ€™s WorldView-3 satellite that can produce spectral images with a resolution down to 0.31m. Technologies to produce images at this resolution have been around for some time, but as reported by Reuters in June, DigitialGlobe has only recently received a license from the US Commerce Department to start selling images with a resolution of up to 0.25m; without this licence they wouldnâ€™t be able to sell this higher resolution imagery.

Regulatory involvement in very high resolution imagery was also demonstrated earlier this year, when in January, the UK government blocked the European Commissionâ€™s effort to set common European regulations on the sale of high-resolution satellite imagery. The UK government currently controls access to data through export licencing conditions on the satellite hardware, and they felt regulations would impact on UKâ€™s ability to export space technology.

Therefore, spatial resolution is an importantÂ term, and one every remote sensing client should understand. Different services require different spatial resolutions, and selecting the most appropriate resolution for your needs will not only ensure that you get exactly what you want, but could also save you money as you donâ€™t want to over-specify.